中华白海豚3个MHC基因座位遗传变异的初步分析

主要组织相容性复合体(Major histocompatibility complex,MHC)基因是由一组紧密连锁的基因组成,是哺乳动物免疫系统中最重要的组成部分.本文选择3个MHC基因座位的第二外元,即:MHC-I类基因和II类基因的DRA和DQB座位,初步调查濒危物种中华白海豚的遗传变异.共鉴定了2个DRA、2个DQB和7个MHC-I等位基因.DRA座位遗传变异非常低,而DQB和MHC-I座位具有相对较高水平的遗传变异.并且,在DQB和MHC-I基因座位的假定的抗原结合位点(Antigen binding sites,ABS),非同义替代明显大于同义替代,提示平衡选择(Balancing selection)维持这两个座位的多态性,而在DRA座位上,并没有检测到平衡选择.系统发生分析表明中华白海豚的MHC等位基因没有聚在一起,而是和其他的物种聚在一起,符合MHC跨种进化(Trans-species evolution)的模式.

A preliminary analysis of genetic variation at three MHC loci of the Indo-Pacific humpback dolphin(Sousa chinensis)

The major histocompatibility complex (MHC) consists of a group of closely linked genes that constitute the most important genetic component of the mammalian immune system. Exon 2 of three MHC loci,i.e.DQB and DRA of MHC class II, and class I, were chosen to preliminarily characterize the genetic variability of the Indo-Pacific humpback dolphin (Sousa chinensis), an endangered species found in coastal China. The DRA, DQB and MHC-I loci each contained two, two, and seven alleles, respectively. Little sequence variation was detected at the DRA locus, relatively higher at the DQB, but considerable sequence variation at the MHC-I. Relatively high rates of non-synonymous (dN) vs. synonymous (dS) substitution in the antigen binding sites (ABS) suggested balancing selection for maintaining polymorphisms at the MHC-I and DQB loci, although this result was not supported by data from the DRA locus. Phylogenetic reconstruction suggested that three MHC loci exon 2 sequences are not separated according to species, but are intermixed with other species, which was consistent with the trans-species evolution model of the MHC.